Clad metal joint closure

ABSTRACT

A plasma arc spray overlay of cladding metals is used over joints between clad metal pieces to provide a continuous cladding metal surface. The technique permits applying an overlay of a high melting point cladding metal to a cladding metal surface without excessive heating of the backing metal.

The present invention relates to clad metal weld joints.

BACKGROUND OF THE INVENTION

Clad metals are composite materials made by mill rolling or explosionforming a thin sheet of a metal that has desired properties, over andmetallurgically bonded at the interface to a base plate of carbon steelor other backing metals. The composite material has the structuralstrength of the backing metal and the corrosion resistance, abrasionresistance, or other useful properties on the clad face. In theweld-joining of the clad metal, it is essential to preserve theproperties of the cladding at the weld joint. Otherwise, the desiredproperties of the clad surface would not be continuous. The propertiesof the backing metal must also be maintained. It is, therefore,necessary to use special weld procedures. In the description herein theterm "cladding metal" refers to the metal layer used for its surfaceproperties in the clad metal seal composite, while "backing" metalrefers to the layer generally used for its structural properties, or foreconomy.

In one welding procedure, the structural backing metal such as steel, isfusion welded using steel for the weld, and the cladding metal sheetportion is welded, using the cladding metal material for a seal closure.In such a procedure, the cladding metal seal closure will becontaminated to a certain extent with the backing metal, although thereis no good test to determine the degree of contamination, and this mayimpair its chemical resistance or other desired properties. This isparticularly true if the welding of the cladding metal requires weldingtemperatures well above the melting point of the backing metal. In viewof this problem, a cover strip of the cladding metal is often placedover the cladding metal closure weld, and welded at its edges to thecladding metal sheet. This type of joint also has limitations in thatthe cover strip is subject to fatigue fracture, and there is nosatisfactory test to evaluate the integrity of the cover stripattachment weld, which welds often prove inadequate in service. Also thefusion welding procedure for attaching the cover strip will heat up thecladding metal surface layer and the backing layer behind it; if thecladding metal is of much higher melting point, the backing metal willbe affected sufficiently to cause contamination of the cladding metal.

In view of the problems encountered in heating the backing metal whilewelding a surface cladding metal, an interlayer of a heat dissipatingmetal may be used. The use of such a layer is most common for compositesin which the cladding metal is of very high melting point, such astantalum or nobium, while the structural backing metal has a much lowermelting point. In closure joints for such a composite, the surface andinterlayers can be machined back from the edges for the pieces to bejoined, and the backing metal of the pieces is joined by a fusionstructural weld. A filler strip of the clad metal material is thenplaced in the recess over the structural weld, with the edges of thestrip abutting the edges of surface and interlayers of the pieces. Acover strip of the cladding metal is then placed over the filler stripand extending beyond it to be attached to the surface of the claddingmetal by welds. Again, there is a problem of the integrity of the weldsattaching the cover strips. There is no good inspection method, andfailures in some types of service are frequent and expensive. Also,cover strips of some metals, e.g. tantalum and niobium, are oftenrelatively thin and flexible and tend to move up and down with changesin temperature and pressure during use. Such movement can result infatigue fracture of the attachment weld.

SUMMARY OF THE INVENTION

The invention involves the use of plasma arc spray overlay forprotection of joint closures between clad metal parts. The inventionfurther involves a method of making clad metal joint closures in whichthe backing metal is joined by a welding procedure and the joint is thencovered by a plasma-arc spray overlay. In a particular procedure inwhich there is a layer of a heat dissipating metal between the claddingmetal and the base metal, the cladding and heat dissipating metals arestripped back from the edges to be joined, so that the base metal can bewelded. An inlay of cladding metal, or cladding metal over heatdissipating metal, is then placed over the welded joint to fill the voidbetween the edges of the cladding metal on the pieces to be joined. Theabutting edges of the cladding metal on the pieces and the inlay arethen joined by welding, using cladding metal material in the welding.The entire joint, including the inlay and the clad metal welds, is thenprotected by a plasma arc spray overlay of the cladding metal material.The method is particularly advantageous for making closure joints wherethe cladding metal is of very high melting point, such as greater than5000° F., and of much higher melting point than the backing metal, as inthe case of tantalum or niobium over steel. The invention is alsodirected to the closure joints which result from the above procedures.

DETAILED DISCLOSURE OF THE INVENTION

In the illustrations, FIG. 1 illustrates the joint closure of thepresent invention with clad metal members with three layers, while

FIG. 2 shows a closure for clad metals with two layers.

FIG. 3 illustrates a prior art procedure employing batten strips. Thefigures will be further described hereinbelow.

Clad metal pieces are prepared for welding by conventional methods. Thecladding metal, e.g. tantalum or tantalum and copper, is machined backfrom the edge of the backing metal a distance of up to one-half inch orso. Referring to FIG. 1, the members A and B with cladding layers 1 and1', 2 and 2', and backing strucures B and 3, with edges 4 and 4', arewelded at such edges with weld 5 composed of the backing metal material.The recess between the edges 6 and 6' of the cladding metals is thenfilled with a filler strip 7 machined to a reasonably close fit betweenedges 6 and 6'. The edges of the filler strip 7 are then welded withcladding metal seal welds 8 and 8' to edges 6 and 6'. The seal welds 8and 8' will not generally penetrate below the upper portion of thecladding metals 1 and 1' and will not penetrate to the surface of thebacking metal 3 and 3'. Moreover, the welds 8 and 8' need not cover theentire length of the butting edges of the filler strip and clad metals,but may be intermittent or stitch welds, but of sufficient strength tohold the filler strip in place. The entire weld closure is then coveredwith the cladding metal at 9 using a plasma-arc spray technique. Plasmaarc spray equipment and techniques can be used as described in AmericanWelding Handbook, Seventh Edition, Vol. 3, Chapter 12, Plasma Spraying,pages 372 through 375.

In the FIG. 1 illustration, the illustrated closure joint isparticularly suitable for composites in which the cladding material 1 istantalum or niobium, the inner layer 2 is copper, and the backing metal3 is steel. Silver also has suitable properties to serve as the heatdissipating inner layer 2, but is much more expensive than copper.Aluminum also has heat dissipating capability, but is inferior to copperin its structural properties.

The structure in FIG. 1 can be utilized with titanium or zirconium asthe cladding metal 1, but it may be preferable with such metals to leaveout the inner layer 2, as titanium and zirconum are not of such highmelting points as to require a heat dissipating inner layer. In such astructure, the filler strip 7 can conveniently be a solid titanium orzirconium piece. The clad titanium and zirconium steels can be producedby explosion bonding processes.

A joint which is of more simple construction is illustrated in FIG. 2 inwhich members A and B have their base metal portions welded at 5 and thecladding metal portions 1 and 1' welded with a weld of cladding metal at10 welding edges 11 and 11' of the cladding metal layer which are fusedto weld 10. The plasma arc spray overlay 9 is then placed over the weld10 and extending inwardly from the edges 11 and 11' to cover a portionof the cladding metal layers 1 and 1'. This type of joint closure mayfind use mainly for clad metals in which the cladding metal is not ofextremely high melting point so that the main advantage of the plasmaarc spray overlay use is the impervious nature of the overlay, ratherthan the capability of applying the overlay without the excessiveheating of the metal.

In a variation of the joint closure illustrated in FIG. 2, the weld 10is omitted and the pieces are machined so that the edges 11 and 11' ofthe cladding metal closely abut each other. Then the plasma arc sprayoverlay 9 can be applied over the joint just as illustrated in FIG. 2.

The plasma arc spray technique is a method whereby a hot gas plasma isused to melt a particulate material and transfer it to a substrate,where it solidifies to become a coating. The plasma is generated bypassing the gas between two concentric electrodes (water cooled) whereit is heated by a sustained high-current, direct current arc.Temperatures within the chamber can reach higher than 30,000° F., whichcauses the gas to expand and issue from the front electrode nozzle ofthe concentric electrodes at a very high velocity. Down stream from thearc, particulate material, e.g. a powder of cladding metal composition,is injected via a carrier gas and mixed with the plasma. Argon isappropriate as a carrier gas for tantalum. The powder melts and iscarried at the gas velocity to the substrate where it is quenched andbonds to the substrate. The plasma-arc spray coatings are characterizedas dense, impervious coatings of high bond strength. The coatings lackcontinuous path porosity and are impervious to gases and liquids. Thuswhen the cladding metal has chemical resistance to particular agents,the plasma arc-spray overlay provides a good seal against suchmaterials.

The plasma arc spray overlay technique does not ordinarily heat thesubstrate to very high temperatures. For example, the substrate maytypically be heated to only 250° F. Thus, aside from the particularmethod of achieving the overlay, it is characterized by having verysmall atomized particles of material, generally well above the fusionpoint, strike a substrate at very high velocity, and under conditions tocause only limited heat transfer to the substrate. Since the plasma arcinvolves ionized gas particles, the recombination of such particlesutilizes heat, thereby lessening the thermal transfer. Utilizing theplasma arc spray overlay as described herein, the rise in temperature ofthe substrate can be kept to less than 500 fahrenheit degrees, oftenless than 300 fahrenheit degrees. While excessive heating of thesubstrate, and particularly its backing metal components, such as steel,can generally be avoided without difficulty with the plasma arc sprayoverlay technique as described herein, it will be understood that thespeed of the deposition or other factors can, if necessary, be adjustedto avoid excessive heating of a backing material. Thus many steels meltaround 2790° F. and it is desirable to keep the temperature of suchbacking materials well below this level to avoid migration of carbon orother materials therefrom to the cladding metal materials.

In plasma arc spraying, the heat is provided by a non-transferred arc.The arc is maintained between an electrode, usually tungsten, and aconstricting nozzle which serves as the other electrode. An inert orreducing gas, under pressure, enters the annular space between theelectrodes where it is heated to a very high temperature (above about30,000° F.). The hot plasma gas passes through and exits from the nozzleas a very high velocity jet. The surfacing material in powder form isinjected into the hot gas jet where it is melted and projected onto thesubstrate.

While plasma arc spraying is particularly useful for coating withmaterials having melting joints above 5000° F., it can be employedusefully with materials of lower melting point. Metals which can be usedinclude aluminum, chromium, copper, molybdenum, nickel, nickel-chromiumalloys, titanium, zirconium, tantalum, niobium and tungsten. Often it isadvantageous to use the same metal for the plasma arc spray overlay asemployed as the cladding metal in pieces being joined, but othermaterials compatible with and bonding to the cladding metal can be used.At times it may be advantageous to use mixtures or allowys including thecladding metal as a component. Various ceramics or oxides such asalumina, titania, zirconia, etc. can be used, as well as Cermets, e.g.alumina nickel and zirconia nickel. Mixtures of components can be usedin various proportions to produce graded (layered) deposits. In generalthe plasma arc spray overlay will be of a surfacing material which iscompatible with the cladding metal to which it is bonded and which formsan impervious, high density coating and which has desirable propertiesmeeting the requirements for which the cladding metal is employed. Inmost cases the overlay surfacing material will be selected to havechemical or corrosion resistance or inertness equal to or better thanthat of the cladding material.

Techniques for electric arc plasma spray coating are at this time welladvanced. They characteristically use a gas stream to contain the arcand constrict the gas stream to collimate the energy of the arc andproduce a high thermal content effluent or plasma, passing powderedcoating material through and with the high thermal content effluent toheat and propel the material and depositing the resulting hot materialon a surface to effect its coating. The deposited material forms asound, dense, adherent surface.

In carrying out the present invention, excesive heating of the claddingsurfaces of the clad materials is to be avoided, so procedures should beused which avoid such heating. Among other factors, heating problems areminimized by not having the clad material in the electrical circuit, andcan also be controlled by the spacing between the plasma torch orificeand the cladding surface, the gas flow rate, and the application time,or work-piece traverse speed. In any event, the plasma arc spray canreadily be utilized in a manner to avoid excesive heating of thecladding surface.

An exemplary procedure for applying material arc spray in implementingthe present invention involves concurrently maintaining a high pressureelectric arc between a non-consumable stick electrode and a secondelectrode spaced therefrom, passing a stream of gas in contact with suchstick electrode to contain such arc, passing such arc-containing gasstream through an orifice which constricts the gas stream andwall-stabilizes a portion of said arc so as to collimate the energy ofsaid arc and gas stream and produce a high pressure arc and high thermalcontent effluent, passing powdered material through and with said highthermal content effluent to produce a high velocity streatm of gas andheated particles, impinging said gas and heated particle stream againstthe cladding and other surfaces of a joint to be overlayed, therebydepositing the so-heated particles on the surfaces as a dense, coherentmass wherein the deposited particles are bound together. Further detailsand exemplifications of plasma arc spray coating procedures which aresuitable for use herein can be found in Gage et al U.S. Pat. No.3,016,447, the disclosure of which is incorporated herein by reference.

Various clad materials are old and well known and used frequently inchemical processing equipment which requires great resistance tocorrosion. It is typical to use a relatively thin layer of a specialpurpose metal or alloy to provide resistance to chemical attack; and touse a lower cost, more common base metal or the structurally supportingbacking. While fabricable and weldable steels are commonly used as thebacking metal, the backing metal can be various steels, includingstainless steel, or nickel-base, copper-base, or cobalt-base materials,or a refractory metal or reactive metal having a composition differentfrom that of the facing material. In the usual applications, thecladding layer will be on only one side of the base metal, but in someapplications the cladding will be on both sides of the base metal, andthe joint closure procedures described herein applying a plasma arcspray overlay over the closure can be employed with such doubly coatedmetal.

Hot mill roll-bonded clad, sometimes called Luken clad, can be employed,and is available with cladding metals including stainless steels, nickeland nickel base alloys, and copper and copper base alloys, and withbacking material of carbon or alloy steel. Stainless clad steels areavailable in gages 3/16 inch to 91/2 inches inclusive, and are generallyspecified in cladding thicknesses between 5 and 20% of the total plategage, depending upon service conditions. In the production of such cladsteels, the cladding metal may be nickel plated prior to rolling inorder to aid in bonding to the backing metal, and to serve as a barrierto carbon migration from the backing steel into the cladding.

Alloys useful in hot mill roll bonded clad include:

Chromium stainless steels: Types 405, 410, 410S, 430A and 430B

Chrominum-Nickel stainless steels: Types 304, 304L, 309S, 310S, 316,316L, 317, 317L, 347, and 318.

Nickel-base alloys: Nickel 200 and 201, Monel Alloy 400, Inconel 600

Copper-base alloys: 90-10 and 70-30 Cupro-nickel

Copper alloys: ASTM B 152, Copper No. 102 and 122

The backing steel most commonly used is A 285, but other steels whichcan be used include:

A 515, grades 55, 60, 65, or 70--service requiring high strength.

A 516--for service down to -50 F.

A 204--for higher strength at elevated temperatures.

A 387--for high strength and resistance to hydrogen.

A 302--for high pressure service and elevated temperature service wherehigh tensile strength are essential.

Further description of clad metals. particularly clad steels, and theircomponents is found in Luken Handbook, Steel Plates and ThinFabrication, pages 251 through 266, the disclosure of which isincorporated herein by reference. The conventional welding aspects ofthe present invention, including weld edge preparation, etc. can be doneas described and illustrated in the cited pages of the Luken Handbook.

A commercial process for explosive bonding of dissimilar alloys in plateform is the Du Pont Detaclad® process. This process permits the bondingof materials which are not amenable to joining by hot mill rolling, andis, for example, used for the cladding of titanium, zirconium, tantalumand niobium to steel. A description of the process is found in U.S. Pat.No. 3,137,937.

The present invention is of special interest for tantalum clad steels,because of the prominence of such materials and the problems presentedin their fabrication. Thus, tantalum has a melting point of about 3000°C. (5430° F.) while mild steel melts at about 1530° C. (2790° F.). Thusin ordinary welding procedures to heat the tantalum to its fusion point,the steel could readily melt and react with the tantalum forming brittleintermetallic compounds. The problems and special conditions involved infabrication of equipment from tantalum clad steels are further describedin U.S. Pat. No. 4,030,849 to H. G. Keifert et al, the disclosure ofwhich is incorporated herein by reference. In particular, the tantalumclad materials useful herein can employ commercially pure unalloyedtantalum, or may use various tantalum alloys, e.g. the Fansteel 63 Metaldescribed in the aforesaid patent.

Clad materials are further described in U.S. Pat. No. 3,443,306 to JohnJ. Meyer, the disclosure of which is incorporated herein by reference.In particular, that patent describes clad materials utilizing a copperinterlayer as a heat dissipating layer during welding procedures, andparticular welding procedures for such clad materials. One of theobjects of the present invention is to provide a method of joining cladmaterial which is an improvement over that of the patent. Tantalum cladmaterials with a copper interlayer are of special interest for formingjoint closures in accord with the present invention, and the cladmaterials disclosed in the U.S. Pat. No. 3,443,306 are suitable forforming such joint closures. As indicated in the patent, the copperlayer is usually of a thickness between 0.050 and 0.070 inch. Theconventional welding techniques described and illustrated in the patentcan be used for welding the base metal layers in the present invention,or for putting fill materials in the recess above the base metal weldand between the edges of the tantalum and copper layers of therespective pieces being joined. Voids around such filter pieces can, ifdesired, be filled with silver welds as illustrated in the Meyer patent,prior to applying a plasma arc spray overlay.

The thickness of the inner cladding layer of clad materials can varywidely, but should be sufficient to provide the desired protection ofthe backing metal and to permit convenient fabrication. With expensivemetals such as tantalum, economic considerations militate againstunnecessarily thick layers. Tantalum or niobium cladding may, forexample, be 0.030 inch thick, and for most uses is apt to be in therange of about 0.025 to about 0.075 inch, or more narrowly, 0.030 to0.045 inch. A commercially available explosive bonded tantalum on steelclad metal has a tantalum layer of nominal 0.030 inch thickness, but thewavy bond interface between the bonded surfaces makes the actualthickness vary.

The plasma arc spray overlay should extend far enough over the joint andadjacent cladding surfaces to insure a continuous impervious claddingsurface and good attachment to the cladding surface. Most overlays willbe 21/2 to 3 inches or more wide and generally of thickness comparableto or greater than that of the cladding metal layer on the compositesbeing joined. Such thickness will usually be or range upwardly fromabout 0.050-0.060 inch or so. The plasma arc spray overlay has theadvantage of being attached across essentially its entire width topresent a good barrier to chemical or other agents contacting a cladmetal surface in a reaction vessel or the like. This contrasts with thebatten strips used in certain prior art procedures, in which a 2 inchwide batten strip would generally have only about 0.010 inch weldattachment from each edge, and in the case of occasional poor weldsmight have as little as 0.002 inch of weld attachment. The comparisonbetween the plasma arc spray overlay and the batten strip technique canbe visualized by substituting a batten strip for the plasma arc sprayoverlay 9 in FIG. 1, but with the weld attachment only being on theouter edges of the batten strip and adjacent cladding surface, with theweld attachment constituting no more than about 10% of the batten stripwidth. This prior art batten strip technique is illustrated in FIG. 3 inwhich batten strip 12 over filler 7 is attached by welds 13 and 13' tocladding metals 1 and 1'. The other illustrated parts are as in FIG. 1.

As a further example of conventional procedures which can be employed inconjunction with the plasma arc spray overlay in accord with the presentinvention, reference is made to Keifert et al U.S. Pat. No. 4,032,243,the disclosure of which is incorporated herein by reference. Asillustrated in that patent, the cladding metal layers can be machinedback from edges to be joined and a channel can be cut into the basemetal to form a recess for a filler insert. The channel can be cuteither before or after welding the base metal portions. Part of thecladding metal layer ends may protrude over portions of the fillerinsert, and an additional filler piece may be inserted between such endsand welded in place with stitch welds. The stitch welds, additionalfiller piece, and adjacent portions of the cladding metal layer may thenbe coated by a plasma arc overlay procedure. The procedure can beemployed with metals having only one cladding metal on a backing metal,or with clad metals having an inner layer for heat dissipation or otherpurposes.

While certain procedures have been illustrated or described herein, itwill be recognized that variations can be used without departing fromthe basic teaching herein. While the invention herein involves astep-wise method to provide impervious closure joint, it will berecognized that conventional techniques are known and available forcarrying out a number of the steps. Thus various procedures are knownfor access to a backer metal, whether one-side or two-side, involvingcutting, machinning or stripping back the cladding metal, and cuttingV-shaped notches and the like in the backing metal to take a weld, aswell as various procedures for putting filler inserts into place. Theplasma arc spray overlay technique is used in conjunction with suchprocedures to apply an impervious coating over the juncture of thejoined pieces, aside from the particular structure of the juncture, in amanner which avoids overheating the backing metal, to form a continuoussurface of the impervious overlay coating with the cladding metal.

What is claimed is:
 1. A clad metal joint closure in which clad metalmembers having a cladding metal surface layer and a backing metal layerare joined by a fusion weld joining opposing portions of the backinglayers, and having an impervious plasma arc spray overlay over the jointbetween the cladding metal surface layers and extending for a shortdistance over the adjacent portions of the cladding metal layers andbonded thereto.
 2. The joint of claim 1 in which the overlay is composedof the same material as the cladding surface.
 3. The joint of claim 1 inwhich the cladding material has a melting point above 5000° F. and thebacking metal is of much lower melting point and the overlay is arefractory material.
 4. A clad metal closure joint in which clad metalmembers are joined which have a very high melting cladding metal surfacelayer, a heat dissipating inner layer, and a structural backing metallayer, in which the backing metal layers are fusion welded together, afiller strip with a cladding metal outer surface is over the fusion weldand between the edges of the cladding and inner layers of the clad metalmembers and welded by attaching welds to the edges of the cladding metallayer; and the filler strip, attaching welds and adjacent portions ofthe cladding metal surface are covered by a plasma arc spray overlay. 5.The joint of claim 4 in which the cladding metal is tantalum, the innerlayer is copper, the backing metal layer is steel, and the plasma arcspray overlay is tantalum.
 6. The joint of claim 4 in which the cladmetal members are an explosion bonded composite of tantalum over copperover steel.
 7. The joint of claim 5 in which the tantalum layer has athickness in the range of about 0.030 inch to about 0.045 inch.
 8. Thejoint of claim 5 in which the tantalum layer has a thickness of about0.030 inch.